Cytoplasmic-Nuclear Male Sterility: Origin, Evaluation and Utilization

Cytoplasmic-nuclear male sterility (CMS), a potential system for economical hybrid seeds production, results from disharmonious interaction between the cytoplasmic and nuclear genetic factors. Evaluation of CMS system for its utility is determined by many factors. Unstable male sterility increases cost and affects quality of hybrid seed production. Cooler weather during panicle development promotes instability in maize and pigeonpea, high humidity in pearl millet and hot temperature of 42 °C and above in sorghum. Effect of genetic background of B-lines on male sterility has been found in case of Al CMS system in pearl millet and sorghum, and fertility restorer gene expressivity in rice. Maintainer gene frequency in germplasm is another factor to be considered which has bearing on genetic diversification of A-lines. In case of Al CMS system in sorghum and pearl millet landraces have greater proportion as restorers. Character association influences the commercial viability of a CMS system. Approaches to estimate the diversity of CMS systems to enhance the exploitation of outstanding inbred lines and also to overcome the risk of disease and pest epidemics, include both traditional field evaluation of hybrids for differential patterns of male fertility restoration and RFLP analysis of mitochondFial DNA. Steps involved in CMS utilization in hybrid cultivar breeding including (i) identification of potential CMS sources, (ii) development of seed parents (A lines) exhibiting complete sterility, (iii) maintenance of seed parents by taking appropriate steps to prevent contamination with foreign pollen, (iv) development of restorer parents through identification of promising restorers, genetic improvement of existing restorers and transfer of restorer genes into high yielding inbred lines and (v) hybrid seed production keeping in view the four key elements - isolation distance, off type removal, flowering synchrony between Aand R-lines, and A:R lines ratio are discussed